1. Field of the Disclosure
This disclosure generally relates to the field of radio communication and, in particular, nondirectional radio beacons.
2. Description of the Art
Radio beacons are used to transmit radio signals as aids to navigation. The signal from a radio beacon is detected by a receiver to allow a location relative to the radio beacon to be determined. Typically, radio beacons transmit on a set frequency and use stationary antennas at a known location. Radio beacons may be directional or non-directional in character. Directional beacons transmit a signal that is stronger in one lateral direction (perpendicular to the length of the antenna) than in another lateral direction. Non-directional beacons transmit a signal that is substantially equal in strength in all lateral directions from the beacon. Non-directional beacons are more commonly used for navigation than directional beacons.
Non-directional beacons transmit a signal equally in all lateral directions, so a receiver placed in any lateral direction from the non-directional beacon will receive approximately the identical signal at the identical strength when positioned at an identical lateral distance from the non-directional beacon. In practice, terrain and other environmental conditions may result in variable signal strength when lateral distances of receivers from the non-directional beacon are identical, even though the antenna of the non-directional beacon is transmitting uniformly in all lateral directions.
The antenna portion of a radio beacon is used to convert electrical signals from a transmitter into electromagnetic radiation, often for the purpose of communication. The transmitter is configured to drive the antenna with an electrical signal within a band of radio frequencies. The design of a specific antenna defines the range of frequencies over which the antenna will operate effectively. The radio frequency spectrum is apportioned by governmental agencies (i.e. Federal Communication Commission) into designated radio frequency bands that may be dedicated for specific uses. Navigational radio beacons in the United States commonly operate in a frequency range of 190 kilohertz to 435 kilohertz, which overlaps with the higher portion of the low-frequency band (30 kilohertz to 300 kilohertz) and the lower portion of the medium frequency band (300 kilohertz to 3 megahertz). The navigation radio beacon frequency range includes frequencies for the Differential Global Positioning System (DGPS) of 283.5 kilohertz to 325.0 kilohertz, and the Global Maritime Distress Network (“NavTex”) of 490 kilohertz to 525 kilohertz. Navigational beacons outside the United States may use frequency ranges that have upper ends that extend higher in the medium frequency band, including about 1800 kilohertz in Brazil. Depending on the construction of the antenna, it may be configured for operation at or around one specific frequency, or the antenna may be designed to operate at several different frequencies.
Generally, the effective operating frequency of a radio antenna is related to the signal wavelength. Since the wavelength is the speed of light divided by the frequency, it can be observed that low frequency antennas may be required to handle very large wavelengths. For example, a radio antenna that is designed to be full signal wavelength and operate at 300 kilohertz, would need to be 1000 meters long (300,000,000 meters/second divided by 300,000 cycles/second). As such large antenna lengths present structural and technical difficulties, most antennas are constructed at only a fraction of the signal wavelength. These smaller length antennas may overcome some structural and engineering problems related to size, but they introduce additional tuning difficulties since the antenna length is different from the wavelength. When an antenna is substantially shorter than the signal wavelength, the antenna is referred to as being “electrically short” or “electrically small.” An electrically short antenna has a longest dimension that is about or less than one-tenth of the signal wavelength. Typical navigational beacon antennas are about 15 to 60 meters in length.
Antenna Tuning
In most antenna systems, the output impedance of the transmitter and the input impedance of the antenna do not match. Transmitters are often installed for antennas without the antennas' parameters being known prior to installation. An antenna tuning unit is used to match the output impedance of the transmitter (as seen by the antenna) with the input impedance of the antenna. By matching the impedances between the transmitter and the antenna, the amount of input energy converted the radio signal is optimized, and the amount of input energy reflected by the interface with the antenna is minimized (VSWR˜1:1). The process of matching of the impedances is called antenna tuning. Maximum power is transferred from the transmitter to the antenna load when the output impedance of the transmitter is the complex conjugate of the input impedance of the antenna.
In order to tune the antenna, an electrical signal is provided from the transmitter. Normal operating power for the transmitter for an antenna, such as a navigational beacon antenna, is substantial enough to result in injury to a radio technician performing the tuning if a fault or failure in the performance of the tuning procedure occurs. To reduce the risk of injury, radio tuning procedures require the radio technicians to reduce the power of the transmitter to a low level for performing the antenna tuning operation. However, injuries may still occur when these procedures are not followed or a malfunction occurs. Once tuned, the antenna will transmit an output signal that is the maximum available based on the frequency and output of the transmitter.
If the transmitter output impedance and the antenna input impedance remain constant, the antenna will not require retuning and configuration of the antenna tuning unit will remain unchanged; however, this is often not the case. Periodic changes in impedances due to environmental changes or relocation of the antenna may require that retuning be performed. While the output impedance of the transmitter is reasonably known due to manufacturing and operating controls, the input impedance of the antenna may vary due to both antenna properties and environmental factors. Antenna properties that may affect input impedance include, but are not limited to, antenna materials and the structural length and shape of the antenna. Environmental factors that may affect input impedance include, but are not limited to, ground resistance, salinity, ground water content, and proximity to standing water.
Matching Networks
The heart of the antenna tuning unit is an impedance matching network. Both tuning and retuning of the antenna involve configuring the impedance matching network between the transmitter and the antenna. The matching network is a circuit that includes impedance elements in series and parallel between the transmitter and the antenna. The impedance elements may comprise circuit components (inductors and capacitors) so that the combination of the output impedance of the transmitter and the impedance of the impedance elements to the matching network is the complex conjugate of the input impedance of the antenna. The impedance elements may be arranged in a variety of configurations, including T-type, Pi-type, and L-type. The configurations are named for the general shape formed by the “legs” or “branches” of the circuit, with each leg representing an impedance element as is known by persons of ordinary skill in the art.
A typical T-type configuration of a matching network includes two inductors in a series branch this divided by a capacitor in a parallel branch. A typical Pi-type configuration of an impedance matching network includes two capacitors in parallel branches on either side of a series branch that includes an inductor. A typical L-type configuration of an impedance matching network includes a series branch with an inductive element (inductor) and a parallel branch with a capacitive element (capacitor), and the parallel branch may be on either side of the series branch.
The impedance elements may be configured to be fixed or variable. Looking more closely at the L-type configuration, for example, the combination of the series inductor and the parallel capacitor will have a resonance frequency, thus the combination will act as a signal filter with the lowest attenuation at the resonance frequency. The inductor and the capacitor may have fixed values, variable values, or there may be one fixed and one variable. In the case of L-type configurations with one or more variable impedance elements, the inductance and/or capacitance can be adjusted. When the inductance/capacitance is adjusted, the resonance frequency will change as well, as will any filtering produced by the impedance matching network. When the input impedance of the antenna load varies, it may be necessary to make adjustments to the impedance matching network in order to maintain maximum signal output.
Maintaining Constant Signal Strength
Another objective of operating a radio beacon is to maintain constant signal strength to the receivers. Even if an impedance matching network is configured to maintain a maximum signal output, the maximum signal output will vary due to changes in the antenna input impedance. In order to maintain constant output signal strength, some radio beacons monitor the amount of electric current delivered to the antenna. By maintaining the amount of electrical current delivered to the antenna at a constant value, generally constant signal strength is provided by the antenna, and it is assumed that a constant electric current delivered to the antenna will result in a consistent signal strength at a specific receiver location. It should be noted that even when the maximum signal output is maintained, the signal strength at the receiver may vary due to atmospheric factors, such as humidity, rain, cloud cover, and solar radiation.
In U.S. Pat. No. 4,951,009, a system is proposed that provides a transmitter-antenna matching network using two variable impedance elements. The system is proposed with L-type, T-type, and Pi-type circuit configurations. At least one of the impedance elements includes a magnetically saturable reactor, such as a tuned transformer with primary and secondary windings wound on a non-linear ferromagnetic core. Both legs of the L-type impedance matching network are shown with inductive and capacitive elements as parts of each of the impedance elements.
In U.S. Pat. No. 4,965,607, a system is proposed that provides a transmitter-antenna matching network using two impedance elements in an L-type circuit configuration. The impedance matching network includes an input series variable inductor and an output parallel variable capacitor.
In U.S. Pat. Pub. No. 2005/0151662, a system is proposed that provides a rescue transceiver that uses received signal strength indication (RSSI) to determine the location of a person buried by an avalanche. The RSSIs of corresponding orthogonal antennas may be compared in order to determine a direction to the signal source, which is located with or near the buried person. The transmitter and the receivers operate independently.
In U.S. Pat. Pub. No. 2013/0033996, a system is proposed that provides a Time Division Multiple Access (TDMA) receiver that uses RSSI feedback. The RSSIs of receiving antennas may be compared to determine which of the antennas is exhibiting superior performance. The system is configured to switch communication from the existing antenna to a superior antenna if it is determined that a superior antenna exists based on the RSSIs.
In U.S. Pat. No. 8,068,798, a system is proposed that provides an auto tuner for antenna matching for an electrically long antenna in a wireless device. The auto tuner uses receiver feedback, transmitter feedback, and current and/or temperature to control a control tuner core. The auto tuner is cycled through predetermined core setups in order to seek out an optimal on-the-air potential regions (coarse), and then a gradient search is performed to fine tune the potentially optimal regions.
In U.S. Pat. Pub. 2007/068512, a system is proposed that provides an integrated spectrum analyzer and vector network analyzer for cellular telephone base stations. The system is configured to transmit signals for testing cellular base stations. The cellular base stations use electrically long antennas and operate in the cellular frequency bands (824 megahertz to about 892 megahertz and 1850 megahertz to about 1990 megahertz).
The above discussed U.S. Pat. Nos. 4,951,009; 4,965,607; and 8,068,798 and U.S. Pub. Nos. 2005/0151662 and 2013/0033996 are hereby incorporated by reference for all purposes in their entirety.
A shortcoming of the systems described above is that none of the prior art proposes a system that uses dual variable inductances without a capacitor in an L-type impedance matching network. Another shortcoming of the systems described above is that none of them use an actual measure of signal strength for controlling antenna output power. Another shortcoming of the systems described above is that none of them incorporate a signal generator into the antenna tuning unit. Another shortcoming of the systems described above is that none of them provide a means of sweeping a frequency range for determining optimal transmission frequency (“sweet spot”) for the antenna. Another shortcoming of the systems described above is that none of them provide a means for testing the antenna tuning unit in the factory or in the field without an operational transmitter.
There is a need for an antenna tuning unit that is configured to match impedances between an output from a transmitter and an input to an antenna that does not require a capacitive element in an L-type impedance matching network. Further, there is a need for an antenna tuning unit that is configured to provide feedback control to a transmitter for regulating output power to maintain constant signal strength at a fixed receiver, where the feedback is based on an actual measurement of signal strength, not delivered power to the antenna. Further, there is a need for an antenna tuning system that provides a signal generator for tuning without the presence of an active transmitter. Further, there is a need for an antenna tuning unit configured to sweep the frequency range to determine a “sweet spot” for transmitting. Further, there is a need for an antenna tuning unity configured for built-in-testing in the factory or the field without an operational transmitter.